Band width and tone control circuit



Dec. 5, 1939. J. wExNBx-:RGER

" BAND WIDTH AND TONE CONTROL CIRCUIT Filed Nov. 9, 1937 Patented Dec.5, 1939 UNITED STATES PATENT OFFICE Julius Weinberger, Bronxville, N.Y., assigner to Radio Corporation of America, a corporation of DelawareApplication November 9, 1937, Serial No. 173,567

7 Claims.

My present invention relates to tone control circuits for radioreceivers, and more particularly to combined band width and tone controlcircuits for radio receivers of the high fidelity type.

In the past it has been customary, in radio receivers, to diminish thereproduction of the higher audio frequencies. Generally, tone controlcircuits have been employed for this purpose, and such circuits havecomprised series resistor-condenser networks shunted across the audioamplifier circuit, the resistor usually being adjustable to produce agradual diminution in the high audio frequency voltage supplied to theloud speaker. Unless the receiver is of the high delity type, the loudspeaker employed usually has a sound output characteristic which isgenerally dish-shaped, and has peaks at about cycles and 200G-3000cycles, assuming a constant applied voltage to the audio network. Whatis most interesting about this type of characteristic is that there isgenerally a high peak in the vicinity of 2000-3000 cycles, with a sharpfalling off in sound output thereafter and with negligible output above5000 cycles.

The result of this peak is to produce brilliancy in the sound output,and to increase the intelligibility of speech. Thus, when a tone controldevice of the type described above is used with a loud speaker havingthe aforesaid characteristic, the peak in the speaker characteristictends to sustain the brilliancy, or intelligibility, of the sound outputeven though the tone control device cuts down the high audio frequenciesfed to the loud speaker. Low frequency boosting is used to improve thebalance of high and low frequencies in the sound output, when a peak isinserted in the high frequency range. When the latter is done, an eectof insufficient bass, or a thin quality, would be apparent in thereproduction of music, if the low frequency end of the response curvewere not raised also.

In high delity radio receivers, the loud speaker which is employedgenerally has a more uniform characteristic covering a wider frequencyrange, usually to about 7500 cycles. Receivers of the high fidelity typehave as their object the faithful reproduction of the audio modulationside bands. More faithful reproduction is secured through the Widefrequency range, and it is undesirable to resort to a peak in thespeaker characteristic at 3000 cycles in order to secure addedbrilliancy in the sound output, as would be the case with loud speakershaving a 5000 cycle cut-off. Such a peak would impair the faith- (Cl.Z50-20) fulness of reproduction when the full audio modulation band issupplied to the loud speaker.

However, receivers of the high fidelity type are usually provided with adevice for restricting its acceptance band. For example, when receivingdistant, or Weak, stations, it is desirable to increase the radiofrequency selectivity of the receiver. This is usually done by employingan adjustable band width control circuit which acts to widen or narrowthe resonance curve characteristic of at least one of the signalselector networks. When using a band width control device, the audiofrequency band supplied to the loud speaker may be limited to 4000 or5000 cycles. The character of the sound output from a high fidelity, orfiat response, loud speaker, supplied With a uniform audio frequencyband limited to only 4000 or. 5000 cycles, is not nearly as pleasing tothe ear as that of a loud speaker output having a dish-shapedcharacteristic of the type described above, when the latter is suppliedwith a similarly limited band of frequencies.

Accordingly, it maybe stated that it is one of the main objects of mypresent invention to provide a tone control circuit for a radio receiverof the high delity type, and which receiver embodies a variable bandwidth control arrangement, the tone control circuit being constructedand arranged in such a manner that a broad peak may be introduced at adesired audio frequency region as the receiver selectivity is sharpenedfor receiving distant, or weak, stations.

Another important object of my invention may be stated to reside in theprovision of a method of controlling the tonal characteristic of areceiver of the high fidelity type, wherein the brlliancy andintelligibility of the audio output is maintained throughout differentconditions of signal reception; the method including the accentuation ofthe high frequency part ofthe audio characteristic simultaneously as thetransmitted radio frequency band is progressively restricted by means ofa variable selectivity'device.

Another object of the invention may be stated to reside in the provisionof a combined adjustable band width control device and audio tonecontrol circuit for a radio receiver of the type having a normally flataudio response when receiving signals of maximum carrier strength; thetone control circuit functioning in a manner such as to accentuate a lowaudio frequency portion and a high audio frequency portion of the audiocharacteristic when the band width is restricted for weak signalreception.

Still another object of the invention may be 55 stated to reside in theprovision of a radio receiver of the high fidelity type, which receiverincludes a tone control circuit vfor providing peaks in the normallyflat audio response characteristic when the radio frequency selectivityof the receiver is increased for weak signal reception; the peaks beingdisposed at 50-150 cycles and 3000- 4000 cycles.

Still other objects of my invention are generally to improvey theefficiency and reproduction of receivers of the high fidelity type, andmore particularly to, provide such a receiver so that it will functionin a desirable manner, and be economically manufactured and assembled.

The novel features which I believe to be characteristic of my inventionare set forth in particularity in the appended claims; the inventionitself, however, as to both its organization and method of operationwill best be understood by reference to the following description takenin connection with the drawing in which I have indicateddiagrammatically a circuit organization whereby my invention may becarried into effect.

In the drawing;

Fig. l schematically shows a radio receiver embodying the presentinvention,

Fig. 2 is an audio characteristic illustrating th operation of theinvention,

Fig. 3 graphically shows the functioning of the band width controldevice.

Referring now to the accompanying drawing, in Fig. 1 there is shown aradio receiving system which is to be understood as being of the highfidelity type. That is to say, the radio frequency circuits up tothedemodulator are all designed to transmit the audio modulation side bandswith maximum efficiencyand with uniform response. The resonant circuitswill, therefore, have yband pass characteristics. The audio amplifiernetworkin such a receiver has a substantially iiat response up to thelimits of the audio range desired to be reproduced; the loud speakeremployed will utilize a substantially flat audio responsecharacteristici Since the receiver can be of any well known type, it isbelieved only necessary to describe the various networks employed in ageneral manner.

The signal collector A is shown as of the grounded antenna circuit type;of course, a radio frequency distribution line, a loop collector, oreveny a collector used on a mobile structure may be employed. Thecollected signals are impressed on a radio frequency amplifier I whichis provided with a tunable selector circuit 2. The ampliiied radiofrequency signals are fed to a first detector 2'; the latter having atunable selector circuit 3. The local oscillator is provided with atunable tank circuit 5, and the locally produced oscillations areimpressed in any desired manner upon the first detector 2. The dottedlines 6 indicate the usual tuner which is employed to varyv the rotorsof the variable tuning condensers in a simultaneous manner. Circuits 2and 3 are tuned to the common signal carrier frequency in the desiredfrequency range, and the latter may comprise the broadcast band of 500to 1500 kc.; whereas, the tank circuit 5 is tunable to a frequency rangewhich differs from the signal range by the operating intermediatefrequency, and the latter I. F. may be chosen from a range of 75 to 450kc.

The I. F. energy delivered by the first detector 2 will be fed throughan IQF. transformer, not shown, to an I. F. amplifier "I, and theamplified I. F. energy is then transmitted through another vcurve ofFig. 3.

I. F. network to the second detector 8. Of course, the receiver need notbe of the superheterodyne type, but can be ofthe tuned radio frequencyamplifier type. In the latter case, the demodulator 8 will be precededby a plurality of cascaded resonant radio frequency amplifiers eachtunable to a common carrier frequency in the desired signal frequencyrange.

The network between I. F. amplifier 'I and the second detector 8 employsa band width control device. The latter may be of any desired type; thespecific device shown is purely illustrative, and acts to vary theaccepted band width in a desirable manner. The primary circuit 9 iscoupled to the secondary circuit ID by means of a link coupling networkII; the circuits lli and 9 are each fixedly tuned to the operating I. F.Coil I2 of link lcoupling circuit II is magnetically coupled, as at lVL,to the circuit 9; coil I3 is magnetically coupled, as at M1 to thecircuit I0. The

ymagnitudes of coupling reactances M-M1 are chosen so that circuits S-Iare coupled with optimum coupling; the latter produces the full lineSuch a curve is a band-pass curve, the modulation side bands aretransmitted to the demodulator with uniform response and the band widthis 20 kc. thereby passing all audio modulation frequencies up to 10,000cycles.

The resistor III is inserted in series with coils I2 and I3; theresistor is variable in magnitude' need not be sharply single peaked asshown; theL full line curve could be narrowed in width to pass a narrowband of frequencies and yet retain the band-pass characteristic. Ofcourse, when the resistor I4 is adjusted to zero, or minimumy value,

then the high fidelity curve of Fig. 3 results.

The control element I5, shown in broken line in Fig. l and designatedband width control, serves to vary the value of resistor I4. Thedemodulator 8 feeds the audio signals to an audio amplifier networkcomprising tube I6 whose inputv` electrodes are coupled across thedetector output circuit. The Aplate of amplifier tube I 6 is coupled tothe input of amplifier I1 through a path including series connectedcondensers I9-2Ii. The

junction of condensers I9 and 2E) is connected toy ground through a pathcomprising three series connected impedances.

The series impedances comprise resistor 2l, a high audio frequencybooster network which includes condenser Z2 andvcoil 23, and a low ire#quency booster network which includes condenser 24 and coil 25. Thenetwork 22-23 is resonant broadly to a frequency at which a peak isdesired in the high audio frequency region, as for example 300D-4000cycles. The Variable resistor 26 is connected in shunt with the reactiveelements of the parallel resonant circuit 22--23. The variable resistor2l is connected in shunt with the reactive elements of the parallelresonant network 2li-25. booster network 2li-25 is tuned to a frequencyat which a peak is desired inthe low audio frequency region, as forexample 50-150 cycles. The

greater the magnitude of each of resistors 26 This char- The lowfrequency and 21, the more pronounced is the boosting '"75 effect of thetwo resonant circuits. On the contrary when the resistors 26 and 21 havea low magnitude, then the two booster circuits are in eectshort-circuited and no boosting results. The dotted lines 30 and 3| areto be understood as designating mechanical couplings between the controlelement I and the adjustable elements of variable resistors 26 and 21.Of course the mechanical coupling 3I may be omitted, and, in that case,only the high frequency booster network 22-26--23 will be employed withthe band width control device I5.

The remaining elements of the audio transmission network are purelyconventional. The amplifier I1 may be the usual power output stage. Theloud speaker I8 is of .the flat audio response type, such as arecommonly employed in receivers of the high fidelity type. In Fig. 2,there are graphically shown the characteristic curves of the audiosystem of the receiver shown in Fig. 1, for the two conditions of highfidelity and high selectivity reception. The full line curve, designatedhigh fidelity, depicts the flat audio response of the audio transmissionnetwork, and it is shown that the response is flat up to substantially'1500 cycles. This is the audio characteristic when the band widthelement I5 varies the magnitude of resistor I4 so as to impart to theselector circuits the full line characteristic curve shown in Fig. 3.When the magnitude of resistor I4 is a minimum, then the magnitudes ofresistors 26 and 21 are also a minimum.

On the other hand when the resistor i4 is adjusted so as to increase theselectivity so that distant, or weak, stations can be properly received,then the coupling elements 30 and 3l are simultaneously actuated toincrease the magnitude of resistors 26 and 21 so that boosting occurs atthe low and high audio frequency portions of the accepted audio range.The dotted line curve in Fig. 2 shows the boosting effect desired whenthe selectivity of the receiver is changed from broad to sharpcondition. Of course the curves in Figs. 2 and 3 are purely pictorial,and those skilled in the art can readily derive the exact relations. Itis to be noted that the mechanical adjusting elements I5, 30 and 3I arelinked, in any well known and desired manner, so that the resistors I4,26 and 21 are conjointly adjustable in a continuous manner from the fullline curves to the dotted line curves of both Figs. 2 and 3.

It may be pointed out that the audio characteristic pictured by thebroken line curve of Fig. 2 produces the best condition for listening toa high fidelity receiver when the band width control has been adjustedto restrict the modulation frequencies to a narrow band. The peak at thehigher audio frequencies creates to some extent the impression of thebrilliancy which exists when wide band reception is used, yet Withoutthe necessity for having decreased I. F. selectivity. Low frequencyboosting is used to improve the balance of high and low frequencies inthe sound output, when a peak is inserted in the high frequency range.When the latter is done, an effect of insucient bass, or a thin quality,would be apparent in the reproduction of fication, as in a separate'rectifier. The vdirect current voltage component of rectified I. F.energy isemployed to decrease the gain of each oflthe R. F. amplifier I,the first detector 2', and I. F. amplier 1. duced with an increase incarrier amplitude and at a rate such that the carrier amplitude at theinput circuit ill is substantially uniform, regard- The decrease in gainis proless of the variation of the carrier amplitude at v the collectorA. that the band width control device and the audio tone controlnetworks are purely illustrative in nature; various other devices can beused in place of the specific ones illustrated. However, in any case, itis desired to correlate the band width control element and the audiotone control circuit in the manner described above.

While I have indicated and described a system for carrying my inventioninto effect, it will be apparent to one skilled in the art that myinvention is by no means limited to the particular organization shownand described, but that many modifications may be made without departingfrom the scope of my invention, as set forth in the appended claims.

What I claim is:

l. A method of operating a radio receiver of the type including a signalselector network and an audio network of the type having a normally fiataudio response; the method including the steps of transmitting signalsthrough said selector network with uniform response and over a wideaudio range for strong signals, transmitting weak signals through theselector network over a limited audio range, and simultaneouslyaccentuating low and high portions of the limited range.

2. In a receiver of the type employing a signal selector network of thevariable band pass type and an audio amplifier having a flat audioresponse over a wide audio range; the method of reception includingsimultaneously restricting the band width passed by said network, forweak signal reception, and accentuating low and high portions of therestricted audio range corresponding to the restricted band width.

3. In a radio receiver of the high fidelity type comprising a band passnetwork, a demodulator and an audio system normally of a fiat audioresponse; means for varying the width of the band passed by said networkthereby to restrict the audio band delivered to said audio system, meansfor providing peaks adjacent low and high frequency ends of therestricted audio band, and control means for simultaneously adjustingsaid two means.

4. In a radio receiver of the high fidelity type comprising a band passnetwork, a demodulator and an audio system normally of a fiat audioresponse; means for varying the width of the band passed by said networkthereby to restrict the audio band delivered to said audio system, meansfor providing a peak adjacent the high frequency end of the restrictedaudio band, and control means for simultaneously adjusting said twomeans, and auxiliary means, responsive to the control means, forproviding a peak adjacent the low frequency end of the audio band.

5. I n a radio receiver of the type comprising a radio frequencyamplifier, detector and audio amplifier; means for varying theselectivity of the radio amplifier between high and low limits; meansfor controlling the response of the audio amplifier; and a unitarydevice for adjusting said two means in a sense such that for high selec-It will be clearly understoodlv rtivity the audio amplifier response ispeaked in a region of 300D-4000 cycles, and additional means for peakingthe audio response in a region of 50-150 cycles, in response toadjustment of the unitary device to high selectivity.

6. In a radio receiver, a high frequency portion provided with a bandpass network, said network being adjustable to selectively transmitsignals vwith high fidelity or high selectivity,` an audio frequencyportion designed to have a luniform frequency response characteristicover a comparatively Wide range when receiving signals with highfidelity adjustment of the band pass network and a uniform frequencyresponse over a narrower range due to attenuation of the higher audiofrequencies when receiving signals with high selectivity adjustment ofthe band pass network, and means for improving the responsecharacteristic of the audio frequency portion when receiving signalswith `the latter adjustment of the band pass network, said meansconsisting of a tone control circuit which accentuates the frequenciesat the low and high frequency ends of said narrower range.

'7. The invention according to claim 6 wherein the tone control circuitcomprises a pair of serially connected resonant circuits, each shuntedby a variable resistance, said resistances being conjointly controlledwith adjustment of the selectivity control of the band pass network.

f JULIUs WEINBERGER.

